Coating systems for automobiles normally comprise a multiplicity of coatings applied to a substrate, typically a steel substrate, which is treated with a rust-proofing phosphate layer, then a cathodic electrocoat primer for additional corrosion protection is applied. A primer (also known as a chip resistant primer, primer-surfacer, or primer filler) is used next to smooth the surface for topcoating and also to provide stone chip resistance to the coating system. Then a top-coat system is applied, sometimes as a single glossy topcoat, more often now as a basecoat with solid color or flake pigments followed by a transparent protective clearcoat, to protect and preserve the attractive aesthetic qualities of the finish on the vehicle even on prolonged exposure to the environment or weathering.
The basecoat and clearcoat compositions are normally applied as a wet-on-wet application to a layer of cured and dried primer. The basecoat layer is applied first and then optionally flashed for a short period to remove at least a portion of the solvent. The clearcoat is then applied to the uncured basecoat and then the basecoat and clearcoat layers are subsequently cured at the same time to form a dried and cured finish. In the conventional method for forming the multi-layer coating film, the underlying primer surfacer layer, however, is cured before being overcoated with basecoat and clearcoat. Historically, cured primers have been used not only to provide a smooth surface on which to apply the topcoat, but also to also prevent interfacial bleeding or intermixing with the overlying basecoat and to avoid disrupting the appearance of the overall topcoat finish. Resistance to intermixing, sometimes referred to as “strike-in” resistance, is especially important for the appearance of glamour metallic finishes which are popular nowadays on automobiles and trucks. Any disturbance of the metallic pigment flake orientation in metallic basecoats after application over the primer-surfacer will detract from the metallic effect of the finish. Therefore, care must be taken to ensure that the metal pigment flakes are not disturbed after painting.
When the paint applied to a substrate contains a metallic effect pigment, the flop index of the dried and cured paint is an important measure of quality. It is important for the metallic effect pigments to orient parallel to the underlying surface to maximize the flop. Generally, basecoats are applied in two thin layers which helps to facilitate the orientation of the metallic flake parallel to the underlying surface. When a majority of the metallic flakes are oriented parallel to the underlying surface, the flop, or brightness change on viewing angle is maximized resulting in a highly desirable effect. To apply two thin layers, an automobile paint facility typically employees a series of two separate spray stations one positioned directly after the other. Since each spray station may use several spray guns, the capital investment for the basecoat spray booth is significant.
In recent years, it has also been strongly desired to reduce the environmental load or impact of automotive assembly plants by reducing VOC (volatile organic compounds) emissions and CO2 (carbon dioxide) emissions generated from operating painting booths and baking ovens. This has led to the use of lower solvent content in the paint and the development of three-layer wet paint systems which make it possible to apply a primer, basecoat and clearcoat wet-on-wet continuously before they are cured all at once in a single bake. With this simplified application process, it is possible to eliminate the separate primer painting booth and primer oven, which also results in substantial cost savings to the automobile manufacturers. The technical hurdles of this process simplification, however, have been significant. For instance, interfacial bleeding and aesthetic appearance, as well as film properties such as chip resistance are still significant concerns.
Attempts have been made to address the forgoing problems by modifying the formulation of the primer coating material. U.S. Pat. No. 6,863,929 of Watanabe et al. describes a method for forming a multilayer automotive coating film using a three layer wet paint process (also referred to as a “3-wet” or a “3-coat-1-bake” process) wherein a standard polyester-melamine primer coating is formulated to also contain acrylic polymer particles, namely in the form of internally crosslinked nonaqueous dispersion (NAD) polymers or internally crosslinked microgel particles. These particles are intended to raise the viscosity and solubility parameter between the primer surfacer and the base coating to prevent intermixing at the interface between the coated layers. However, use of such particle-filled systems also suffers from some drawbacks.
For example, the microparticles also tend to create voids in the surface of the wet primer where the basecoat can still flow in and intermix, resulting in defects in the aesthetic appearance such as loss of smoothness, gloss, head on brightness, and/or metallic effect. Sagging of these coatings, especially on vertical panels, such as doors, fenders, rocker panels, etc, is also a problem. These particle-filled systems are also not able to maintain dry film builds at normal commercial levels. Film builds must therefore be reduced to allow the NAD or microgel particle to migrate to the interface. Yet, thin films are an impediment as they tend to subject the underlying corrosion-protective electrocoated primer layer to excessive UV light transmission and deterioration. Thin films or thin film regions are also inadequate for mechanical properties and visual appearance of the overall finish.
Therefore, there is still a need to find a more effective way to prevent the intermixing of the primer surfacer and basecoat and clearcoat layers when applied in a wet-on-wet-on-wet (i.e., a 3-wet) manner and make it possible to eliminate the primer baking process and reduce the environmental impact of the coating system, while also maintaining film builds, the overall appearance such as high gloss and distinctness of image and film properties of the coating system.